It is well known that the molecular weight of ethylene polymers can be controlled by the use of chain transfer agents. The much preferred chain transfer agent employed by the art is hydrogen and for a number of good reasons. It is a relatively cheap gas that is easily fed in controlled concentrations with ethylene to the reactor. Other choices of chain transfer agent include metal alkyl compounds. They are disadvantaged by having to be added to the reactor as liquids, typically dissolved in volatile solvents, which can introduce a volatility problem to the reaction system. As a result, they are more difficult to meter to the reaction. They are also more expensive than hydrogen and they contribute metal contaminant to the ethylene polymer. Most importantly, hydrogen is a superior chain transfer agent.
It is also well known that chain transfer agents affect ethylene polymerization catalyst activities. However, this effect differs among the chain transfer agents and is dependent upon the selected catalyst system. Chain transfer agents can reduce catalyst activity, and the extent of such reduction is dependent upon the chain transfer agent chosen and the catalyst system. Higher activity catalysts can be correlated with higher productivities of ethylene polymer. So, it follows that when chain transfer agents are used, an objective is to achieve as high a level of catalyst activity as possible within the framework of the molecular weight targets of the polymerization reaction. A chain transfer agent system that enhances the activity of the catalyst over that achievable with hydrogen would have significant value to the art.
There is a family of high activity vanadium catalysts that have been described for the polymerization of olefins such as ethylene and .alpha.-olefins, that are based on the use of a supported reduced and complexed vanadium halide catalyst precursor. Illustrations of these catalysts can be found in Beran et al., U.S. Pat. No. 4,508,842, patented Apr. 2, 1985. Beran et al. described an ethylene polymerization catalyst comprising a supported precursor of a vanadium halide/electron donor complex and alkylaluminum or boron halides to form a reduced vanadium catalyst precursor, which when combined with alkylaluminum cocatalyst and alkyl halide promoter, provides enhanced polymerization and productivity plus a superior polyethylene product. According to Beran et al., the process involves polymerizing ethylene with or without at least one C.sub.3 to C.sub.10 alpha-olefin monomer in the gas phase at a temperature between about 30.degree. C. to about 115.degree. C. wherein the monomers are contacted with a catalyst composition comprising a vanadium complex and a modifier which are impregnated on a solid, inert carrier. Beran et al. differentiate by the use of a supported precursor, a cocatalyst and a promoter in which the supported precursor comprises a vanadium halide-electron donor reaction product and modifier impregnated on a solid, inert carrier. The halogen in the vanadium halide is chlorine, bromine or iodine, or mixtures thereof. A particularly preferred vanadium halide is a vanadium trihalide, such as vanadium trichloride, VCl.sub.3. The electron donor is a liquid, organic Lewis base in which the vanadium halide is soluble. The electron donor is selected from the group consisting of alkyl esters of aliphatic and aromatic carboxylic acids, aliphatic ketones, aliphatic amines, aliphatic alcohols, alkyl and cycloalkyl ethers, and mixtures thereof. Preferred electron donors are alkyl and cycloalkyl ethers, such as tetrahydrofuran ("THF"). Between about 1 to about 20, preferably between about 1 to about 10, and most preferably about 3 moles of the electron donor are complexed with each mole of vanadium used.
The disclosure of Beran et al. is incorporated herein by reference. The vanadium catalysts of Beran et al. are hereinafter characterized as the "Beran et al. Catalyst System."
Cozewith et al., U.S. Pat. No. 4,540,753, patented Sep. 10, 1985, disclose a process for making EPR and EPDM resins or elastomers utilizing a catalyst system that comprises a hydrocarbon-soluble vanadium compound in which the vanadium valence is 3 to 5 and an organo-aluminum compound, at least one of the vanadium compound/organo-aluminum pair selected must also contain a valence-bonded halogen. The vanadium compounds are vanadium oxy halide, vanadium halides and complexes of a Lewis base capable of making hydrocarbon-soluble complexes with VCl.sub.3, such as tetrahydrofuran, 2-methyltetrahydrofuran and dimethyl pyridine. Illustrative examples of vanadium compounds are vanadyl trihalides, alkoxy halides and alkoxides such as VOCl.sub.3, VOCl.sub.2 (OBu) where Bu=butyl, and VO(OC.sub.2 H.sub.5).sub.3, with the most preferred vanadium compounds being VCl.sub.4, VOCl.sub.3, and VOCl.sub.2 (OR). The patent's cocatalyst is preferably an organo-aluminum compound such as AIR.sub.3, Al(OR')R.sub.2, AlR.sub.2 Cl, R.sub.2 Al--O--AlR.sub.2, AlR'RCl, AlR.sub.2 I, Al.sub.2 R.sub.3 Cl.sub.3, and AlRCl.sub.2, where R and R' represent hydrocarbon radicals, the same or different. The most preferred organo-aluminum compound is cited to be an aluminum alkyl sesquichloride such as Al.sub.2 Et.sub.3 Cl.sub.3 or Al.sub.2 (iBu).sub.3 Cl.sub.3. The reaction is effected in at least one mixfree reactor, with essentially one active catalyst species, using at least one reaction mixture which is essentially transfer agent-free, and in such a manner and under conditions sufficient to initiate propagation of essentially all of the copolymer, chains simultaneously, wherein the copolymer chains are dispersed within the reaction mixture. According to the patentees,
Chain transfer agents for the Ziegler-catalyzed polymerization of alpha-olefins are well known and are illustrated, by way of example, by hydrogen or diethyl zinc for the production of EPM and EPDM. Such agents are very commonly used to control the molecular weight of EPM and EPDM produced in continuous flow stirred reactors..sup.1 FNT .sup.1 It is to be noted that Cozewith et al. describe polymerization in a reaction system where initial catalyst surge is not a problem. PA0 "However, at the least the last part of the reduction is carried out using zinc metal or alkyl zinc compounds of the formula R.sub.2 Zn where R is as described above. The preferred material is elemental zinc in a fine powdered form." PA0 straight chain acyclic dienes such as: 1,4-hexadiene, 1,6-octadiene, and the like; PA0 branched chain acyclic dienes such as: 5-methyl-1,4-hexadiene, 3,7-dimethyl-1,6-octadiene, 3,7-dimethyl-1,7-octadiene and the mixed isomers of dihydro-myrcene, dihydroocinene, and the like; PA0 single ring alicyclic dienes such as: 1,4-cyclohexadiene, 1,5-cyclooctadiene, 1,5-cyclododecadiene, and the like; PA0 multi-ring alicyclic fused and bridged ring dienes such as: tetrahydroindene, methyltetrahydroindene, dicyclopentadiene, bicyclo(2,2,1)-hepta-2,5-diene, alkenyl, alkylidene, cycloalkenyl and cycloalkylidene norbornenes such as 5-methylene-2-norbornene (MNB), 5-ethylidene-2-norbornene (ENB), 5-propyl-2-norbornene, 5-isopropylidene-2-norbornene, 5-(4-cyclopentenyl)-2-norbornene, 5-cyclohexylidene-2-norbornene, and the like. PA0 a supported precursor containing a high activity vanadium catalyst complexed with an electron donor and reduced with a modifier; PA0 an aluminum cocatalyst; PA0 a metal alkyl chain transfer agent where the metal is from Groups 2 and 12 of the Periodic Table Of The Elements, and PA0 a halogenated organic promoter.
There is substantial literature indicating the creation of a catalytically active vanadium by the reduction of vanadium halides to a reduced, viz. divalent, state. Carrick et al., JACS, vol. 82, p. 1502 (1960) describe the reduction of VCl.sub.4 to the divalent state for of a vanadium ethylene catalyst utilizing conventional reducing agents, such as triisobutylaluminum and zinc alkyls. Karol et al., JACS, vol 83, pp. 2654-2658 (1961) discuss the partial and total reduction of vanadium halides such as VCl.sub.4 to divalent structures and the catalytic activity resulting with respect to the polymerization of ethylene to polyethylene.
Jacob et al., Z. anorg. allg. Chem., 427, pp. 75-84 (1976) illustrate the complexity of such reduction reactions in the presence of THF. From the teachings of Beran et al., the resulting divalent vanadium compounds are complexes which include THF in the structure.
Cumulative to the above, Smith et al., U.S. Pat. No. 4,559,318, patented Dec. 17, 1985, describe a number of procedures for making VX.sub.2, where X is halogen, which involves the reduction of VX.sub.4 or VX.sub.3 by reaction with reducing agents followed by the complexation of the VX.sub.2 with an ether such as THF. Such is effected on a support surface. Also, Smith et al. discloses that in forming the catalyst, once the catalyst is combined with the support, the combination is subjected to a reducing agent "using a hydrocarbon soluble reducing agent such as a dialkyl zinc compound . . . " Smith et al. emphasized reduction with zinc in the following passage to be found at column 3, lines 49-59:
According to the patent, the catalysts are useful for the polymerization of ethylene alone or with a variety of 1-olefin monomers to make homopolymers and copolymers. Illustrative of the 1-olefin monomers are one or more of propylene, 1-butene, 1-pentene, 1-hexane and 1-octene.
Roling et al., U.S. Pat. No. 4,434,242 teach chemisorption of a V.sup.(V) oxyhalide, such as O=VCl.sub.4, vanadium alkoxide or VCl.sub.4 onto a metal oxide support having its surface hydroxyl groups reacted away with a substantially stoichiometric amounts of Group 13.sup.2 metal (i.e., Al, Ga) alkyl, followed by further treatment with an alcohol to narrow the molecular weight distribution of the resultant polymer. FNT .sup.2 New notation of the Periodic Table Of The Elements, see Chemical and Engineering News, 63(5), 27, 1985); as noted in CRC Handbook of Chemistry and Physics, 67th Edition, CRC Press Inc., Boca Raton, Fla., inside frontcover.
A number of significant problems have been noted with the fluid bed operability of high activity vanadium catalysts, including the vanadium catalyst encompassed by the Beran et al. Catalyst System, in the polymerization of ethylene and .alpha.-olefins to produce elastomeric polymers, such as ethylene-propylene copolymers (EPR). These problems are oftentimes characterized by the formation of polymer chips, chunks, sheets and lumps in the fluid bed, which in some cases can lead to sudden defluidization. Another set of problems can occur with high activity vanadium catalysts on starting up of the fluid bed and/or during transitioning of the reactor. For example, startup of a gas-fluidized polymerization reactor goes through a sensitive stabilization period due to impurities trapped in the reaction system. Low (ppm) level impurities have a deactivating influence on the catalyst and contribute to polymer particle adhesion. The net effect is that layers of polymer fines containing high concentrations of catalyst are formed on reactor surfaces and in places where mixing forces are reduced. When polymerization is then initiated, localized hot spots can result, with consequent chunking and eventual reactor shutdown.
Although such difficulties have occurred in a variety of ethylene polymerization operations with such catalysts, they have been most pronounced under EPR operating conditions, especially under EPM operating conditions, where there are relatively high concentrations of propylene in the reactor and a low-crystallinity resin is being produced. In this case, these difficulties are believed to stem from a number of contributing factors, such as (i) the magnitude of the initial kinetic spike in the standard catalyst reaction profile, which is much greater with propylene as a comonomer than with .alpha.-olefins higher than propylene and (ii) the elastomeric nature of the EPM resin being produced, which can soften, become sticky and agglomerate due to the increase in temperature associated with hot spots and reaction surges.
Important variables in influencing the degree of stickiness leading to more or less agglomeration are the polymerization reaction temperature and crystallinity of the polymer being produced. Higher temperatures increase the propensity to form agglomerates, and less crystalline polymers, such as ultra low density polyethylene, ethylene/propylene copolymers (EPM), and ethylene/propylene/diene monomer (EPDM), usually display a greater tendency to agglomerate. EPM and EPDM polymers having a density less than 0.88 g/cc are noted for their capacity to soften and agglomerate.
Those polymerization conditions which result in stickiness and agglomeration of the polymer are termed "polymerization conditions that normally would yield an undesirable amount of agglomerated polymer with high activity vanadium catalysts," in order to characterize this invention so as to compensate for the variety of reactants, polymerization conditions and catalyst compositions encompassed herein.
Elastomeric ethylene-alpha-C.sub.3 -C.sub.18 olefin copolymers encompass ethylene-propylene copolymers (EPR) (inclusive of EPM or EPDM copolymers), ethylene-butene copolymers, and the like. Illustrative of such polymers are those comprised of ethylene and propylene or ethylene, propylene and one or more dienes. Copolymers of ethylene and higher alpha-olefins such as propylene often include other polymerizable monomers, such as non-conjugated dienes, illustrated by the following:
Of the non-conjugated dienes typically used to prepare these copolymers, dienes containing a double bond in a strained ring or .alpha. position, are preferred. The most preferred dienes are 5-ethylidene-2-norbornene (ENB) and 1,4-hexadiene. The amount of diene, on a weight basis, in the copolymer can range from about 0% to about 20% with about 0% to about 15% being preferred. The most preferred range is 0% to 10%.
The preferred EPR copolymers are copolymers of ethylene-propylene (EPM) or ethylene-propylene-diene (EPDM). The average ethylene content of the copolymer could be as low as about 10% on a weight basis. The preferred minimum is about 25%. A more preferred minimum is about 30%. The maximum ethylene content may be about 85% on a weight basis. The preferred maximum is about 80%, with the most preferred maximum being about 75 weight % ethylene.
High activity vanadium catalysts would be desirable for EPR products because they achieve efficient comonomer incorporation in the polymers with relatively random distribution of the comonomers in the polymer structure. However, their above-noted deficiencies for producing EPR polymers in a fluid bed, such as producing a high initial surge in the polymerization rate which causes exothermic temperature excursions that can soften the polymer in the course of polymerization and foul the bed by virtue of resin agglomeration, thereby degrading fluid bed operability, has impaired their use for such applications. The initial surge is strongest when propylene is one of the comonomers polymerized with ethylene and is present in relatively high concentrations. The problem is magnified in EPR copolymers because of their low softening temperature. Thus, there is a need for a high activity vanadium-based catalyst system that can produce EPR-type polymers at acceptable productivity levels without inducing agglomeration. There is also a need for a high activity vanadium catalyst system that is effective in producing a variety of ethylene-containing polymers with low softening temperatures.
From the various studies done, a correlation is seen to exist between initial kinetic surge in the reaction and agglomeration of polymer particles in the bed. It is thus concluded that moderation of the initial kinetic surge establishes the catalyst system's capability of moderating the agglomeration problem. It would be most desirable to have a catalyst system for the polymerization of ethylene to homopolymers and copolymers, that not only reduces the kinetic surge in the polymerization, but also achieves reasonable superior activities, thereby enhancing the possibility of greater production of polymer of good quality.